Third-generation mobile systems (3G) based on WCDMA radio-access technology are being deployed on a broad scale all around the world. A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), giving a radio-access technology that is highly competitive. In order to be prepared for further increasing user demands and to be competitive against new radio access technologies, 3GPP introduced a new mobile communication system which is called Long Term Evolution (LTE). LTE is designed to meet the carrier needs for high speed data and media transport as well as high capacity voice support into the next decade. The ability to provide high bit rates is a key measure for LTE. The work item (WI) specification on Long-Term Evolution (LTE) called Evolved UMTS Terrestrial Radio Access (UTRA) and UMTS Terrestrial Radio Access Network (UTRAN) is finalized as Release 8 (Rel. 8 LTE). The LTE system represents efficient packet-based radio access and radio access networks that provide full IP-based functionalities with low latency and low cost. The detailed system requirements are given in 3GPP specification TR 25.913, “Requirements for Evolved UTRA and Evolved UTRAN”, ver. 9.0.0, freely available at www.3gpp.org.
In LTE, scalable multiple transmission bandwidths are specified such as 1.4, 3.0, 5.0, 10.0, 15.0, and 20.0 MHz, in order to achieve flexible system deployment using a given spectrum. In the downlink, Orthogonal Frequency Division Multiplexing (OFDM) based radio access was adopted because of its inherent immunity to multipath interference (MPI) due to a low symbol rate, the use of a cyclic prefix (CP), and its affinity to different transmission bandwidth arrangements. Single-carrier frequency division multiple access (SC-FDMA) based radio access was adopted in the uplink, since provisioning of wide area coverage was prioritized over improvement in the peak data rate considering the restricted transmission power of the user equipment (UE). Many key packet radio access techniques are employed including multiple-input multiple-output (MIMO) channel transmission techniques, and a highly efficient control signaling structure is achieved in Rel. 8 LTE.
The overall architecture is shown in FIG. 1 and a more detailed representation of the E-UTRAN architecture is given in FIG. 2. The E-UTRAN comprises eNBs, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the UE. The eNB hosts the Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC), and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane. It performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated UL QoS, cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of DL/UL user plane packet headers. The eNBs are interconnected with each other by means of the X2 interface. The eNBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (S-GW) by means of the S1-U. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and eNBs. The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PDN GW). For idle state UEs, the SGW terminates the DL data path and triggers paging when DL data arrives for the UE. It manages and stores UE contexts, e.g., parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception.
The MME is the key control-node for the LTE access-network. It is responsible for idle mode UE tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the SGW for a UE at the initial attach and at the time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the HSS). The Non-Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to UEs. It checks the authorization of the UE to camp on the service provider's Public Land Mobile Network (PLMN) and enforces UE roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming UEs.
The downlink component carrier of a 3GPP LTE system is subdivided in the time-frequency domain in so-called subframes. In 3GPP LTE each subframe is divided into two downlink slots as shown in FIG. 3, wherein the first downlink slot comprises the control channel region (PDCCH region) within the first OFDM symbols. Each subframe consists of a given number of OFDM symbols in the time domain (12 or 14 OFDM symbols in 3GPP LTE (Release 8)), wherein each OFDM symbol spans over the entire bandwidth of the component carrier. The OFDM symbols thus each consist of a number of modulation symbols transmitted on respective NRBDL×NscRB subcarriers as also shown in FIG. 3.
Assuming a multi-carrier communication system, e.g., employing OFDM, as for example used in 3GPP Long Term Evolution (LTE), the smallest unit of resources that can be assigned by the scheduler is one “resource block”. A physical resource block is defined as NsymbDL consecutive OFDM symbols in the time domain and NscRB consecutive subcarriers in the frequency domain as exemplified in FIG. 3. In 3GPP LTE (Release 8), a physical resource block thus consists of NsymbDL×NscRB resource elements, corresponding to one slot in the time domain and 180 kHz in the frequency domain (for further details on the downlink resource grid, see, for example, 3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)”, version 8.9.0 or 9.0.0, section 6.2, available free of charge at http://www.3gpp.org and incorporated herein by reference). The term “component carrier” refers to a combination of several resource blocks. In future releases of LTE, the term “component carrier” is no longer used; instead, the terminology is changed to “cell”, which refers to a combination of downlink and optionally uplink resources. The linking between the carrier frequency of the downlink resources and the carrier frequency of the uplink resources is indicated in the system information transmitted on the downlink resources.
Cell search procedures are the first set of tasks performed by a mobile device in a cellular system after initial power-up. It is only after the search and registration procedures that a mobile device is able to receive and initiate voice and data calls. A typical cell search procedure in LTE may involve a combination of carrier frequency determination, timing synchronization and identification of unique cell identifier. These procedures are typically facilitated by specific synchronization signals transmitted by the base station (BTS). However, these synchronization signals are not continuously used in connected modes for a mobile device. Hence, only minimum resources in terms of power, subcarrier allocation and time slice are allocated for synchronization signals.
The cell search procedure enables the UE to determine the time and frequency parameters which are necessary to demodulate the downlink and to transmit uplink signals with the correct timing. The first phase of the cell search includes an initial synchronization. Accordingly, the UE detects an LTE cell and decodes all the information required for registering to the detected cell. The procedure makes use of two physical signals which are broadcast in the central 62 subcarriers of each cell, the primary and secondary synchronization signals (PSS and SSS, respectively). These signals enable time and frequency synchronization. Their successful detection provides a UE with the physical cell-ID, cyclic prefix length, and information as to whether FDD or TDD is employed. In particular, in LTE, when a terminal is switched on, it detects the primary synchronization signal, which for FDD is transmitted in the last OFDM symbol of the first time slot of the first subframe (subframe 0) in a radio frame (for TDD the location is slightly different, but still well-determined). This enables the terminal to acquire the slot boundary independently of the chosen cyclic prefix selected for the cell. After the mobile terminal has found the 5 millisecond timing (slot boundaries), the secondary synchronization signal is looked for. Both the PSS and SSS are transmitted on 62 of the 72 reserved subcarriers around the DC carrier. In the next step, the UE shall detect a physical broadcast channel (PBCH) which, similarly to the PSS and SSS, is mapped only to the central 72 subcarriers of a cell. The PBCH contains the Master Information Block (MIB) including information about the system resources. In LTE up to Release 10, MIB had a length of 24 bits (14 bits of which are currently used and 10 bits are spare). MIB includes information concerning the downlink system bandwidth, physical HARQ Indicator Channel (PHICH) structure, and 8 most significant bits of the System Frame Number (SFN).
After successful detection of the master information block (MIB), which includes a limited number of the most frequently transmitted parameters essential for initial access to the cell, the terminal activates the system bandwidth, meaning that it has to be able to receive and detect signals across the indicated downlink system bandwidth. After acquiring the downlink system bandwidth, the UE may proceed with receiving further required system information on the so-called System Information Blocks (SIB). In LTE Release 10, SIB Type 1 to SIB Type 13 are defined, carrying different information elements required for certain operations. For instance, in case of FDD the SIB Type 2 (SIB2) includes the UL carrier frequency and the UL bandwidth. The various SIBs are transmitted on a Physical Downlink Shared Channel (PDSCH) and thus (cf. details to PDSCH and PDCCH below) the respective allocations are assigned by a Physical Downlink Control Channel (PDCCH). Before the terminal (UE) is able to correctly detect such (or any) PDCCH, it needs to know the downlink system bandwidth from the MIB.
The above-mentioned cell identity (cell-ID) will identify the cell uniquely within the PLMN. The cell identity is a global cell-ID that is used to identify the cell from an Operation and Maintenance (OAM) perspective. It is transmitted in the System Information and is designed for eNodeB management within the core network. The global cell identity is also used for UE to identify a specific cell in terms of RRC/NAS layer processing. Physical cell identity is the cell identity at physical layer. The physical cell identity has a range of 0 to 503, and is used to scramble the data to help the mobile separate information from the different transmitters. A physical cell identity (cell-ID) will determine the primary and secondary synchronization signal sequence. It is similar to the Scrambling Codes from UMTS. There are 504 unique physical-layer cell identities. The physical-layer cell identities are grouped into 168 unique physical-layer cell-identity groups, each group containing three unique identities. The grouping is such that each physical-layer cell identity is part of one and only one physical-layer cell-identity group. A physical-layer cell identity NIDcell=3NID(1)+NID(2) is thus uniquely defined by a number Ng in the range of 0 to 167, representing the physical-layer cell-identity group, and a number NID(2) in the range of 0 to 2, representing the physical-layer identity within the physical-layer cell-identity group.
Synchronization signal is composed of a primary synchronization signal (PSS) and secondary synchronization signal (SSS). The sequence used for the primary synchronization signal is generated from a frequency-domain Zadoff-Chu sequence according to NID(2). By detecting primary synchronization signal, NID(2) could be detected. The sequence used for the second synchronization signal is an interleaved concatenation of two binary sequences with a length of 31 bits. The concatenated sequence is scrambled with a scrambling sequence given by the primary synchronization signal. The SSS sequences are based on maximum length sequences, known as M-sequences, which can be created b cycling through every possible state of a shift register of length n. This results in a sequence of length 2^n−1. In particular, the two 31-bit long binary sequences to be concatenated are such M-sequences. For further details on the primary and secondary synchronization signal, see, for example, 3GPP TR 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 12)”, version 12.1.0, section 6.11, available free of charge at http://www.3gpp.org and incorporated herein by reference.
After receiving the PPS and SSS, the timing is adapted by the receiving UE. In particular, the UE synchronizes its receiver to the downlink transmission received from the synchronization source (eNB). Then, the uplink timing is adjusted. This is performed by applying a time advance at the UE transmitter, relative to the received downlink timing, in order to compensate for propagation delays varying for different UEs. The timing advance procedure is described concisely in Section 18.2.2 of the book “LTE The UMTS Long Term Evolution: From theory to practice”, 2nd edition, ed. By S. Sesia, I. Toufik, M. Baker, Wiley, 2011.
Proximity-based applications and services represent an emerging social-technological trend. Current and intended uses include services related to commercial services and Public Safety that would be of interest to operators and users. The introduction of a Proximity Services (ProSe) capability in LTE would allow the 3GPP industry to serve this developing market, and will, at the same time, serve the urgent needs of several Public Safety communities that are jointly committed to LTE.
Device-to-Device (D2D) communication is a technology component for LTE-A, Release 12. The Device-to-Device (D2D) communication technology allows D2D as an underlay to the cellular network to increase the spectral efficiency. For example, if the cellular network is LTE, all data carrying physical channels use SC-FDMA for D2D signaling. The “D2D communication in LTE” is focusing on two areas; Discovery and Communication. In D2D communication, UEs transmit data signals to each other over a direct link using the cellular resources instead of through a base station (BS, eNodeB, eNB). D2D users communicate directly but may remain controlled under the network, i.e., at least when being in coverage of an eNB. Therefore D2D can improve system performances by reusing cellular resources. It is currently assumed that D2D operates in uplink LTE spectrum (in the case of FDD) or uplink subframes of the cell giving coverage (in case of TDD except when out of coverage). Furthermore D2D transmission/reception does not use full duplex on a given carrier. From individual UE perspective, on a given carrier D2D signal reception and LTE uplink transmission do not use full duplex, i.e., no simultaneous D2D signal reception and LTE UL transmission is possible. Further current working assumptions concerning the radio access for D2D of LTE are described in 3GPP TS 36.843, v c.0.1, “Study on LTE Device to Device Proximity Services; Radio Aspects” (in the following referred to as “TS 36.843”), freely available at www.3gpp.org.
In D2D communication when UE1 has a role of transmission, UE1 sends data and UE2 receives it. UE1 and UE2 can change their transmission and reception role. The transmission from UE1 can be received by one or more UEs like UE2. FIGS. 4 and 5 illustrate the protocol layers, service points and multiplexing in downlink and uplink respectively for the transmission on different channel types.
It was agreed in 3GPP RAN1 as a working assumption that a synchronization source is any node that transmits a D2D synchronization signal (D2DSS). It can be an eNB or a normal UE. When the synchronization source is eNB, the D2DSS is the same as Rel-8 PSS and SSS. D2D UE uses the synchronization signal(s) to determine the timing for transmitting D2D signal. It was also agreed as a working assumption that before starting to transmit D2DSS, a D2D UE scans for synchronization sources. If a synchronization source is detected, the UE may synchronize its receiver to it before it may transmit D2DSS. If no synchronization source is detected, a UE may nevertheless transmit D2DSS. A UE may (re)select the D2D Synchronization Source it uses as the timing reference for its transmissions of D2DSS if the UE detects a change in the D2D Synchronization Source(s), based on following metrics:                Synchronization source type. eNB or UE        Received D2DSS quality        Number of hops from eNB.        